Friday, March 01, 2013

Bubbles support \(10\GeV\) or \(50\GeV\) dark matter

March 2013 is expected to be a great dark matter month, especially due to the eagerly expected results from AMS-02 that may emerge as early as the next week (ANTARES has seen nothing a few days ago). Joseph S. has brought my attention to an excellent astro-ph paper by Tracy Slatyer (IAS) and Dan Hooper (FNAL)

that eliminates all doubts that the authors belong among the very top of the world's astroparticle physics. They looked at the spectrum of the Fermi bubbles – that Tracy co-discovered – and decided to write down the most natural model(s) that explain(s) the observer spectrum. What the models depend upon – and what the observations should therefore clarify – is what is the spectrum of electrons, the radiation, and masses and dominant decay channels of hypothetical dark matter particles that team up to produce the spectrum.

I think that they show their ability to split the data into regions that seem to be dominated by different effects, explain the partial datasets, and design economic theories that are able to explain several features of some partial datasets simultaneously. What does it mean in practice?

In practice, it means that they divided the picture of the galaxy to two regions, according to the latitude (angular distance from the galactic center). For the "tropical" and "polar" regions, they found two different spectra and two different models that explain them.

What are they?

Far from the galactic plane, at least 30 degrees from the equator, they see that the spectrum ceases to depend on the latitude. The gamma rays over there may apparently be described by inverse Compton scattering. Note that the inverse Compton scattering is the process \[

e^-+\gamma\to e^-+\gamma

\] in which the photon's energy increases (the photon steals some energy from the initially fast electron): that's why it's inverse. This description works if the electrons obey a power law spectrum and if they collide with an interstellar radiation field. Cutely enough, this model may also explain the microwave "haze" assuming that the electrons are moving in a magnetic field whose intensity is a microgauss or so.

For the "tropical" region, they need a different description and a different mechanism, however. The reason is that the spectrum seems to have a peak near \(1-4\GeV\) if \(E^2\,\dd N/\dd E\) is graphed against \(E\). The inverse Compton scattering is no good to produce such peaks. Instead, they decide that this part of the dataset is similar to results previously reported from the Galactic Center. The luminosity per volume seems to decrease as the \(r^{-2.4}\) power law with the distance from the Galactic Center.

The chief NASA administrator and a rapper gives a not terribly comprehensible but sufficiently stringy introduction to dark matter.

An important part of the answer is that this radiation seems to be consistent with the annihilation of dark matter particles! It's either due to \(10\GeV\) dark matter particles pair-annihilating into lepton pairs or due to \(50\GeV\) dark matter particles annihilating into quark-antiquark pairs. They seem to propose two comparably likely scenarios for the possible mass and dominant interactions of the dark matter particles.

Note that none of these two scenarios should be new to the TRF readers. A possible \(10\GeV\) dark matter particle has been discussed many times in the context of the "Is Dark Matter Seen" war between the direct search experiments. The allies in the "Dark Matter Is Seen" coalition do generally claim that they see collisions with numerous \(10\GeV\) or sub-\(10\GeV\) dark matter particles. The "Dark Matter Is Not Seen" axis is vehemently rejecting all these assertions.

However, even dark matter particles around \(50\GeV\) have previously been spotted by careful TRF readers. In January 2012, I mentioned that Virgo favored a \(20-60\GeV\) dark matter particle. This was based on a fresh preprint by Han et al. who looked into Virgo, Fornax, and Coma clusters through the Fermi satellite and concluded something remarkably similar to Slatyer and Hooper: there is either a \(20-60\GeV\) dark matter particle annihilating into \(b\bar b\) quark pairs, or a \(2-10\GeV\) dark matter particles annihilating into \(\mu^+\mu^-\).

Because this Han et al. paper doesn't seem to be cited by Tracy and Dan, you could view the agreement as a strong piece of circumstantial evidence that the possible masses and dominant annihilation channels of the dark matter are pretty much what Slatyer and Hooper say now. This sociological argument has only one delicate problem, aside from its being sociological and therefore worthless: in July 2012, Han et al. II wrote a new paper in which they added several new point sources. It seems they thought that they had to add them. Their original signal got contaminated and all the peaks of "extended emission" went away.

Now, another half a year later, Slatyer and Hooper are reviving a statement very similar to the Han et al. statement from January 2012. Cosmology entered a high-precision era 15 years ago but even when it depends on solid fellow disciplines such as astroparticle physics, you may see that it sometimes takes a lot of time to pick the right answers in similar uncertain situations and to choose the winners in assorted dark matter wars.

In these wars, March 2013 could turn out to be an analogous month to June 1944. I hope that some readers know what the D-Day means ;-) – the Battle of Stalingrad wasn't good enough for the previous sentence because it was too long and its date was therefore too fuzzy. At any rate, stay tuned. Things could get very exciting and very convincing very soon.

11 comments:

This is getting really exciting! It’s about time we observed the major matter constituent of the universe.I do think you meant to say “microgauss” rather than “milligauss” fields in order for cyclotron radiation to account for the microwave haze.

Wow, Gene! First, thanks, and second, your catching of the milli/microgauss error is amazing. Was it due to your pure on-top-of-your-head knowledge of this physics or were you actually studying the abstract/paper? ;-) I suspect it's the former and one should stay breathless. Needless to say, I have no intuition about the difference between microgausses and milligausses in similar phenomena. But yes, if you ask me and force me to think, it is quite certainly true that the energy density from an astronomical scale milligauss field would be too much.

A milligauss field seemed too large intuitively so I did a very rough calculation, which confirmed it. I have done a lot of work with magnetic fields, including my PhD thesis, so they are very familiar to me.

For the most motivated models with this light new particle, your complaint is solid and it is a genuine contradiction. However, one may think of models in which the dark matter particle, while light, is so weakly interacting with the quarks and gluons and electrons - that made up the existing and current colliders - that only an undetectably small number of these particles are produced during the career of the accelerators.

Dear jitter, the two lobes don't represent the ordinary density of matter (mass density). They represent a more complicated thing, one could say "just an optical illusion". Certain radiation appears in the lobes which doesn't mean that this is where the mass is exclusively concentrated.

If the dark matter halo had this infinity-symbol-like shape, there could still be things orbiting in between. However, their orbits would be unstable, trying to get caught by one of the lobes. In fact, the whole situation would be unstable - the lobes would probably wish to merge in some way, unless stabilized by some extra force someone would have to propose first.

Even so, the Anomalous Single Photon experiment at PEP should have been exquisitely sensitive to a 10 GeV dark matter particle that annihilates to a lepton pair. At 29 GeV, PEP would have been sitting just above the onset of pair production. Radiative processes (particularly off the t-channel propagator) would show this resonance as a sharp cutoff in the high-transverse-momentum single photon spectrum around 9 GeV, just the sort of thing ASP was designed to see with negligible background. In other words, they were looking right at the sweet spot for the exactly the right signature, and saw nothing. Granted, the coupling must be small, but not so small (well away from any resonance) as to make the Fermi bubbles disappear.